1. Field of the Invention
The present invention relates to a method and an apparatus for measuring bromate ions, more specifically for measuring bromate ion concentration in sample water.
2. Description of the Related Art
Raw city water such as river water contains bromide ions (Br−). Upon performing ozonation on the raw city water, the bromide ions react with ozone to produce bromate ions (BrO3−). Bromate ions are considered to be a carcinogenic substance. In view of this, the World Health Organization (WHO) sets the guideline value for bromate ion concentration in drinking water to be 10 μg/L. The Japanese government revised the ministerial ordinance on the water quality criteria published on May 30, 2003 to set the standard value of bromate ion concentration in city water to be 10 μg/L.
Ion chromatograph-post-column absorptiometry (the IC-PC method) has been known as a method for measuring bromate ion concentration in water. The IC-PC method is a method for quantifying bromate ions by separating bromate ions in sample water using an anion-exchange column, adding sulfuric acid and a mixed solution of sodium nitrite and sodium bromide to an eluate of the bromate ions, thereby converting the bromate ions into tribromide ions, and measuring the ultraviolet absorbance of the tribromide ions. In this IC-PC method, a two-step reaction occurs, requiring that the first-step reaction convert bromic acid into tribromide ions with a potassium bromide/sulfuric acid and the second-step reaction ensure the linearity of a calibration Line in the low-concentration range using a sodium nitrite solution. Given this situation, the measurement operation for the bromate ion concentration by the IC-PC method is complicated and is hence hard to be adapted to process equipment.
In view of such a background, developed recently is a method for measuring bromate ion concentration using fluorescence intensity. In this method, trifluoroperazine (TFP), which is a fluorescent substance that reacts upon coexistence with bromate ions, and hydrochloric acid are added to sample water, its fluorescence intensity is measured at an excitation wavelength of 300 nm and an emission wavelength of 480 nm, and a fluorescence intensity difference with a reference sample that contains no bromate ion is calculated. Using a calibration line between fluorescence intensity difference and bromate ion concentration, bromate ion concentration is measured from the calculated fluorescence intensity difference. This method can measure bromate ions simply, quickly, and with high precision.
TFP exhibits a quenching reaction when the excitation wavelength and the measurement emission wavelength are 300 nm and 480 nm, respectively. However, when the excitation wavelength is 300 nm and the measurement emission wavelength is 480 nm, the optimum hydrochloric acid concentration that ensures the linearity of a calibration line is as extremely high as 6 N. In a conventional method, because the hydrochloric acid concentration for use in measurement is high, equipment is likely to corrode, and running costs increase. In this measurement condition, bromate ion concentration may not be measured accurately because the slope of the calibration line changes due to coexisting nitrate ions. In view of such a background, anticipated is a technology that can measure bromate ion concentration with high precision without being affected by the coexisting substance while reducing hydrochloric acid concentration required for measurement.
For the foregoing reasons, there is a need for a method and an apparatus for measuring bromate ions that can measure bromate ion concentration with high precision while reducing hydrochloric acid concentration required for measurement.